Page 23 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 17

Figure 1.9 A trap where the oil bearing formation has moved up from its original
place of deposition.
fractures. The pressure required to create a fracture is termed “frac-
ture pressure.”
Fracture pressure is expressed as either:
• A pressure—psi, bar, or kPa.
• A fluid gradient—psi/ft, Bar/m, or kPa/m.
• A fluid weight equivalent—ppg, kg/l, or SG.
Knowing the fracture pressure is essential for workover and intervention
operation, as exceeding fracture pressure would lead to severe fluid loss and
a consequent loss of the hydrostatic overbalance. Fluid loss to the formation
also carries a risk of formation damage, and the severe losses associated with
a fractured formation are very damaging. The impact on productivity is
likely to be severe. Most operating companies will have policy and proce-
dures in place to ensure that fracture pressure is not accidentally exceeded
during completion and workover operations. However, there are occasions
when fracturing is a required part of the intervention. Fracture pressure is
deliberately exceeded during the installation of frac-pack sand control com-
pletions. It is also routinely exceeded during acid fracturing and propped
frac stimulation operations.
Fracture pressure is related to the weight of formation matrix (rock
and sediments), and the fluid occupying the pore spaces above the zone
of interest. These two factors combine to produce what is termed “over-
burden pressure.” Although the density of the overlying formation varies